FIX: Incorrect Results from Mixed-Type Expressions

MS-DOS                   | OS/2

The information in this article applies to:

• Microsoft FORTRAN for MS-DOS, versions 4.0, 4.01, 4.1, 5.0, and 5.1
• Microsoft FORTRAN for OS/2, versions 4.1, 5.0, and 5.1

SYMPTOMS

A program compiled with Microsoft FORTRAN version 4.0, 4.01, 4.1, 5.0, or 5.1 under MS-DOS or version 4.1, 5.0, or 5.1 under OS/2 can generate incorrect results on mixed-type expressions when an INTEGER variable is used on both sides of an equation. The /Od compiler option, which is used to suppress optimization, has no effect.

CAUSE

The compiler is incorrectly converting intermediate values to the INTEGER type used on both sides of the arithmetic statement.

RESOLUTION

To avoid this problem, keep data types consistent within arithmetic expressions by using the intrinsic functions to force the conversion of intermediate expressions to the proper data type.

STATUS

Microsoft has confirmed this to be a problem in Microsoft FORTRAN versions 4.0, 4.01, 4.1, 5.0, and 5.1 for MS-DOS and versions 4.1, 5.0, and 5.1 for OS/2. This problem was corrected in FORTRAN PowerStation.

MORE INFORMATION

Sample Code 1

The following code reproduces the problem:

      integer*4 i, j
      i=10
      j=i-1.1   ! correct (10.0 - 1.1 = 8.9, truncated to 8)
      i=i-1.1   ! incorrect   (10 - 1 = 9)

      write(*,*) 'j=',j,' i=',i      ! outputs j=8 i=9
      end

Sample Code 2

The following code illustrates the solution:

      integer*4 i, j
      i=10
      j=i-1.1         ! correct (10.0 - 1.1 = 8.9, truncated to 8)

      i=real(i)-1.1   ! convert i to real, then set to an integer
                      ! variable when assigned
      write(*,*) 'j=',j,' i=',i      ! correct output generated
      end